The 29-30 November 2016 Northern Alabama Tornado Outbreak, Part 2: Radar, Profiler, and In-Situ Observations of the Role of Topography in Supercell and Tornado Environmental Evolution

A destructive late-fall tornado outbreak impacted portions of the southeastern United States on 29-30 November 2016. The outbreak featured nearly 40 tornadoes and was responsible for six direct fatalities. The northeastern corner of Alabama served as the epicenter of the outbreak, with five tornadoes (two significant tornadoes (EF2-EF3 intensity on the Enhanced Fujita Scale), one killer tornado, and four direct fatalities) from four separate supercells or line segments in a two-county area.

As part of the Verification of the Origins of Rotation in Tornadoes Experiment – Southeast (VORTEX-SE) field campaign, the University of Alabama in Huntsville’s Severe Weather Institute – Radar and Lightning Laboratories (UAH-SWIRLL) has sought to investigate the role of topography in the evolution of severe storm and tornado behavior. The Sand Mountain plateau in northeastern Alabama is a particular focal point of interest for these studies owing to a propensity for enhanced tornado activity over this region, particularly for enhanced tornadogenesis on the northwestern side of the plateau.

In anticipation of the severe weather event reaching northeastern Alabama on 29-30 November, UAH-SWIRLL dispatched three instrument platforms to Sand Mountain and the adjacent Tennessee River valley. The Rapidly-Deployable Atmospheric Profiling System (RaDAPS), which features a 915-MHz Doppler wind profiler, a 35-channel microwave profiling radiometer, a ceilometer, and surface instrumentation, was deployed atop the Sand Mountain plateau to retrieve wind and thermodynamic profiles from atop the plateau. The Mobile Doppler Lidar and Sounding system (MoDLS), which features a Doppler wind lidar and surface instrumentation, was deployed in the Tennessee Valley for high-resolution retrieval of flows within the valley and upstream of Sand Mountain. The Mobile Alabama X-band (MAX) radar was placed atop Sand Mountain for high-resolution single-Doppler observations and to potentially pair with the Weather Surveillance Radar – 88 Doppler (WSR-88D) at Hytop, Alabama (KHTX), also located in the northeastern part of the state. Additionally, two mobile sounding teams were deployed, one at the MAX site atop Sand Mountain and one at the MoDLS site in the Tennessee Valley. The four tornadic storms all passed within approximately 20-50 km of the instrument array, allowing for the profiling array to act as a high-resolution proximity profiling array. Of the five tornadoes, three were in extremely rough terrain in far northeastern Alabama, with total along-track elevation changes of up to 360 m, while the two remaining tracks occurred on the Sand Mountain plateau, with one descending approximately 270 m into a cove. No tornadoes were observed in the Tennessee Valley in between the rough terrain to the north and Sand Mountain to the south, despite the mesocyclones within two of the tornadic supercells having passed over the valley with considerable residence time for each.

In this presentation, we detail the environments observed by the UAH-SWIRLL instrument platforms during the tornado outbreak. The behaviors of the tornadic storms as observed by scanning radar and ground and aerial survey analysis are compared to the environmental observations to refine preliminary hypotheses of the roles of terrain in severe storm and tornado evolution in northeastern Alabama. We characterize noted changes in the environments atop Sand Mountain versus within the Tennessee Valley, particularly changes in the wind profiles observed at these locations, and discuss how these environmental changes may be related to observed changes in storm characteristics. These preliminary results and refined hypotheses are placed in the broader context of the VORTEX-SE 2017 dataset to define a path forward in expanding the knowledge of how terrain and general boundary layer heterogeneity impact severe storm evolution.